This invention relates to wide-angle microwave lens for line source radar antenna applications and in particular to such lens which permit a resulting radiated beam to be scanned in small spaced increments while the array factor remains essentially constant.
Wide-angle microwave lens used as an antenna line source have been known for a long time. One such wide-angle microwave lens has been described in U.S. Pat. No. 3,170,158 for "Multiple Beam Radar System" by Walter Rotman and has come to be known as a Rotman type lens antenna. A typical such lens is comprised of a pair of flat parallel conducting plates which comprise an RF transmission line fed by means for injecting electromagnetic energy into the parallel plate region, a plurality of coaxial transmission lines connected to output probes which extract energy from the parallel plate region, and a linear array of radiating elements fed individually by the coaxial transmission lines radiating energy into space. The physical location of the means for injecting electromagnetic energy into the parallel plate region along a focal arc determines the angle of a beam radiated by the antenna. If the means for injecting is traversed along the focal arc the radiated beam will scan through the antenna field of view. It has been proposed to use a Rotman lens antenna in a microwave landing system (MLS) where the antenna is used to sweep a radiated beam through space at a known rate through known bounds. Thus, an aircraft periodically illuminated by the radiated beam could determine from the characteristics of the illumination its position in space with respect to the radiating antenna. If the radiated beam is swept horizontally then the aircraft could determine its azimuth with respect to the radiating antenna, while a beam swept vertically would provide elevation information to the aircraft, as known to those skilled in the art. Usually one antenna is arranged to sweep a beam vertically, thus providing, for practical purposes, simultaneous azimuth and elevation information to an illuminated aircraft.
The means for injecting has taken the form of a plurality of feed probes positioned along so as to define the focal arc. When the various feed probes are energized so as to feed electromagnetic energy into the parallel plate region one at a time consecutively, the resulting beam will scan through space in distinct steps whose angular separation is directly related to the angular separation between adjacent feed probes. It is desirable, of course, that the aforementioned steps be as small as possible since positional uncertainty at the illuminated aircraft increases as the angular separation between consecutive beams, and hence distance between adjacent feed probes, increases. In short, a smoothly commutated beam provides the best degree of positional certainty at the illuminated aircraft, thus dictating relatively close feed probe spacing. However, if the feed probes are positioned too close to one another adjacent probes will be parasitic to an energized probe, thus distorting its resulting beam shape. One means of providing a well shaped smoothly commutating beam is through the use of a single feed probe instead of the above described plurality and physically scanning the single probe along the focal arc of the lens. This type of scanning probe, however, requires an undesirable mechanism to produce the mechanical motion.